1. Field of the Invention
The invention is directed generally to compositions and methods for controlling vascular tension in extremities to improve blood flow at extremities and control pulmonary blood pressure. More specifically, the invention is directed to the control of ATP release by red blood cells, via manipulating phosphodiesterase 3 in red blood cells using inhibitors of phosphodiesterase 3.
2. Description of the Related Art
It is known in the art that shear stress in arterioles stimulates the production of endothelium relaxation factors that act on the smooth muscle cells of arterioles, causing the vessels to relax and permit increased blood flow. An example of an endothelium relaxation factor is nitric oxide. Work performed by the inventors and collaborators have suggested that red blood cells, while under conditions of low oxygen tension or cell deformation (e.g., as RBCs squeeze through smaller vessels), release ATP into the blood vessel lumen. The ATP released from red blood cells, in turn leads to NO synthesis and release of endothelium derived relaxation factors by the endothelial cells, which enables vasorelaxation.
It has also been shown by the inventors that patients with certain diseases produce red blood cells which release none to subnormal levels of ATP in response to low oxygen tension or under mechanical deformation. Those diseases include (but are certainly not envisioned to be limited to) diabetes, cystic fibrosis, hyperinsulinemia, type 2 diabetes, and primary pulmonary hypertension. The inventors envision that RBCs are an important system for the regulation of blood flow into areas and conditions of low oxygen tension, and subject to mechanical deformities such as in capillary beds and at extremities. The invention discloses (a) the mechanism by which RBCs can release ATP, (b) compositions to enhance the production of ATP by RBCs, and (c) methods of treating diseases such as pulmonary hypertension and diabetic blood flow problems.
The erythrocyte, by virtue of the hemoglobin that it contains, has long been recognized as a vehicle for oxygen (O2) transport. In addition to this well established role for the erythrocyte in the circulation, it has been shown that this cell can also participate in the regulation of vascular resistance via the release of ATP [2-8]. The erythrocyte releases ATP when exposed to reduced O2 tension or mechanical deformation, as well as in response endogenous mediators [1, 2, 4, 7, 9, 11, 13]. This erythrocyte-derived ATP has been shown to be a stimulus for NO synthesis [3, 7, 8]. The ability of the erythrocyte to release ATP in response to physiological stimuli enables this cell to control its own distribution within the microcirculation and, thereby, to regulate O2 delivery [2-4, 6]. Indeed, it has been proposed that the erythrocyte, via its ability to release ATP in response to reduced O2 tension, produces local vasodilation in areas of skeletal muscle with increased O2 demand resulting in the matching of O2 delivery with metabolic need [2, 5, 6, 30].
Recently, the inventors have defined a signal transduction pathway that relates physiological and pharmacological stimuli to ATP release from erythrocytes [12]. This pathway includes the heterotrimeric G proteins Gi and Gs, adenylyl cyclase (AC), cyclic adenosine monophosphate (cAMP), protein kinase A (PKA), and the cystic fibrosis transmembrane conductance regulator (CFTR) (FIG. 10) [9-13]. It is important to note that, in this pathway, activation of either Gs or Gi results in the stimulation of AC activity and cAMP synthesis [9, 10]. The finding that activation of Gi is capable of stimulating some AC subtypes is not unique to the erythrocyte [31, 32]. It has been reported that, in multiple cell types, AC type II is activated by the βγ subunit of Gi [10, 12, 14, 31, 32]. We have shown that both Gi and AC type II are components of human erythrocyte membranes [10, 14].
As depicted in FIG. 10, increases in cAMP are required for ATP release from erythrocytes [11]. The level of cAMP in a cell is the product of its synthesis by AC and its degradation by phosphodiesterases (PDEs) [33]. In addition to AC II, it is known in the art that human (any and all) erythrocytes possess PDE activity, however, neither the PDE subtypes present nor their regulation have yet to be fully characterized [12, 14, 34-38].